The invention relates to an improvement of a tone control device such as a graphic equalizer.
In a conventional tone control device, as shown in FIG. 1, the outputs of bandpass filters 1.sub.1 through 1.sub.n having corresponding transfer functions B.sub.n (s) are applied to the input and output amplifiers 3 and 4 through variable resistors 2.sub.1 through 2.sub.n, respectively, which have grounded center taps so that the gains at a plurality of particular frequencies are boosted or cut. This conventional tone control device is disadvantageous in that the quality of signals is lowered as the gain is varied due to the non-parallel arrangement of the filters.
In another prior art arrangement, a tone control device was obtained by forming a circuit which had the following transfer function T(s): ##EQU1## where s is the Laplace operator, .omega..sub.0 is the center angular frequency, Q is the sharpness, k.sub.1 is the attenuation factor at the angular frequency .omega..sub.0, and k.sub.2 is the amplification factor at the angular frequency .omega..sub.0. With this transfer function, if the values k.sub.1 and k.sub.2 are varied with the values .omega..sub.0 and Q held constant, then a flat frequency characteristic can be obtained with k.sub.1 =k.sub.2 =1, a boosted frequency characteristic with k.sub.1 =1 and k.sub.2 &gt;1, and a cut frequency characteristic with k.sub.1 &gt;1 and k.sub.2 =1, all as shown in FIG. 4.
A prior art circuit arrangement which was intended to cover the audio frequency band by combining circuits having transfer functions T.sub.1 (s) through T.sub.n (s) having different center frequencies is shown in FIG. 5. However, this arrangement suffers from noise problems and an increased distortion factor.
Accordingly, an object of this invention is to provide a tone control device in which the bandpass filters are connected in parallel with one another and the frequency characteristics of the device can be varied without lowering the quality of signals.
An object of this invention is also to provide a tone control device in which band-stop filters are connected in parallel and tone control in plural channels is effected while the device is free from noise and has a low distortion factor.